Whenever I hear people talking about replacing the foundation, I usually hear about the expense of jacking the house up costing an extra ten thousand or thereabouts. I've wondered why it's not possible to replace the foundation walls 1 or 2 at a time and replace the floor with the last wall? Wouldn't this eliminate the need for the ten thousand dollar expense of jacking the house up?

From my understanding, when a wall is being replaced, they put support jacks under the beams to hold the house up then demolish and build a new wall with new footings. Is that correct? Is there a risk in the foundation not being level by doing it this way? Am I missing something?

I think it depends on type of foundation and where you live. If you have walls between foundation and floor, you could remove the wall above. Out here on the hills we often have one side of the foundation full height. Then the engineers want the floor joist landing on that wall so the floor actually helps support the wall from pushing in. Poring concrete to full height would be very hard although, there are ways that it can be done.

#1 The foundation needs to act as a one piece unit when built of it's going to crack at the butt joints. There would be no way to have a running bond, the blocks would have to be stacked, making a very weak foundation.
The footing needs to be pored all at one time so the there is no weak spots where it will settle and crack. Old concrete will not bond well to new.

#2 Without lifting the house there's no way to slide in the top blocks and tap them down so there even.

I have red clay block foundation built in 1919. It hasn't moved a lot, it's bowed in about 1" on two walls. A few moisture issues that I've attempted to correct with grading and new gutter & downspout system.

This foundation has no rebar to my knowledge and it doesn't have a sump pump.

There's areas where I'd like to add posts underneath the floors for additional support and to reduce the bounce.

I guess I didn't realize that it weakens the foundation to replace the walls one or two at a time. There's a guy in my area who rebuilds concrete walls and he gave me a bid to replace two of them. The bid included going 2 feet deeper, rebar, damp-proofing, and the installation of drain tiles and a sump pump.

So would it be correct then for me to say that doing this would result in a weaker foundation and would do more harm than good?

Basically, I was just worried that my foundation is weak, crumbling, and inferior and I wanted to improve it.

Right now things are so tight and job so far apart to get It it's likly the so called foundation guy will go along with whatever you suggest to get a paying job.
Any foundation made in sections is subject to cracking. In a clay soil even more so.

I would pay for a geo-engineer to look over the plan and make his suggestion and inspection when the job is being done.
I'm not sure how you save much money here when you still have to jack up to change each wall seperately.

Spend the money on the engineer. You could have six guys like Joe and I or your foundation guy and you will have six differrent answers. Up here I think the engineer can be held responsible for twenty years.

When I had a few foundation contractors look at my situation they didn't say that the house would have to be lifted up just supported by temporary posts. I think it would transfer the weight to the basement floor.